Wavelength selective optical filter

ABSTRACT

The present invention provides a wavelength selective optical filter device ( 240 ) for receiving input radiation and outputting corresponding filtered output radiation, characterized in that the filter device ( 240 ) includes a plurality of at least partially mutually coupled Fabry-Perot optical resonators ( 330, 340, 360; 360, 400, 430 ) for filtering the input radiation to generate the output radiation, the filter device ( 240 ) being tunable from a first radiation wavelength to a second radiation wavelength by mutually detuning the resonators in a period where the resonators are being retuned from the first wavelength (λ 1 ) to the second wavelength (λ 2 ) so that the filter device ( 240 ) is substantially in a non-responsive state during the period. The resonators incorporate freely suspended mirrors ( 360, 430 ) which are electrostatically actuated to affect tuning of the resonators ( 330, 340, 360; 360, 400, 430 ). The filter device ( 240 ) is thereby capable of tuning between different wavelengths without tuning through wavelengths therebetween. The filter device ( 240 ) can be included into an add-drop filter ( 10 ) for providing channel add and drop functions when the filter ( 10 ) is incorporated in a multichannel WDM communication system ( 100 ).

This application is a communication of 09/734,495 now U.S. Pat. No.6,449,403, filed December 11, 2000, now allowed.

The present invention relates to a wavelength selective optical filter,in particular but not exclusively to a wavelength selective opticalfilter which is also operable to provide a controllable degree ofattenuation.

In contemporary optical communication systems, wavelength divisionmultiplexing (WDM) techniques are used. Such techniques enable manychannels bearing communication traffic to be multiplexed onto radiationpropagating along a guided optical path, for example an optical fibre.Each channel has associated therewith an allocated range of wavelengthswhich are used to convey the communication traffic associated with thechannel. Thus, WDM techniques allow increased exploitation of opticalfibre bandwidth in order to satisfy future demand for enhanced datarates in communication networks.

Use of WDM in contemporary communication systems has created a need fordevices which can be connected to optical paths thereof conveying WDMcommunication traffic, the devices operable to extract communicationtraffic corresponding to a specific channel without interfering withcommunication traffic conveyed in other channels; radiationcorresponding to these other channels is transmitted through the devicessubstantially unmodified. Such devices are known as add-drop filters.

There arises a further requirement in contemporary optical communicationsystems using WDM techniques and incorporating add-drop filters for thefilters to be reconfigurable, namely for the filters to be retunable toselect different channels. Moreover, it is a yet further requirementthat the communication systems should be reconfigurable whilst inoperation conveying communication traffic. Thus, each add-drop filterneeds to be retunable from a first selected channel to a second selectedchannel without tuning through channels intermediate between the firstand second channels and causing traffic conveyed in these intermediatechannels being interrupted or disturbed during retuning. A number ofconventional add-drop filters have been reported in the literature andsold commercially which are capable of being tuned from one channel toanother. Such conventional add-drop filters incorporate optical filterswhich tune continuously; as a consequence, they cause disturbance ofcommunication traffic on intermediate channels when being reconfigured.Such optical filters are implemented in a number of ways, for example ascascaded Mach-Zehnder filters fabricated as silicon planar waveguidesand as micro-mechanically tuneable Fabry-Perot filters. A U.S. Pat. No.5,739,945 describes a single cavity continuously-tuneable optical filterincorporating electrostatically actuated mirrors.

Tunable optical filters are known in the prior art.

For example, a U.S. Pat. No. 4,240,696 describes an optical filterincluding a plurality of adjacent layer pairs, each pair having anincident and an emerging surface. Each pair further comprises a firstdielectric layer, a second dielectric layer and a control electrodedisposed between and in contact with the layers. The filter additionallyincludes a plurality of ground electrodes disposed on the layer pairs toelectrically contact each incident and emergent surface, a source ofelectrical potential, and a switch for connecting the source between thecontrol electrodes and the ground electrodes. In the filter, opticalradiation is reflected by the filter upon closing the switch and therebyapplying the electrical potential in opposite directions across thefirst and second layers. Electrodes of the layer pairs are thusconnected in parallel so that the pairs are not capable of beingmutually independently tuned. Moreover, there is no basis in the contextof the invention for it to be advantageous to make the pairsindependently tunable.

Moreover, in a further example, a U.S. Pat. No. 5,170,290 describes hightotal transmission tunable comb filter structures. The structurescomprise moderately thick layers of optical material having periodicrefractive index modulation features comprising a multiplicity ofcoherently-coupled, weakly-resonant optical cavities. The structures arecharacterised by spectra of at least order 5 relative to a fundamentallowest-order cavity resonance consisting of narrow, moderate to highdensity reflection lines occurring in one or more sets, each set beingcharacterised by lines equally spaced by wave number if opticaldispersion is neglected. Filters provided by such structures can beelectro-optically or mechanically tuned such that the peaks within aspectral band of interest shift by one harmonic order to reflect ortransmit optical radiation of any specific wavelength within a band. Thecavities are not capable of being mutually independently tuned in theembodiments described in the patent. Moreover, there is no basis in thecontext of the invention for it to be advantageous to make the cavitiesindependently tunable.

A first approach to providing add-drop filters which do not tunecontinuously in contemporary systems involves demultiplexing andremultiplexing techniques. Use of such techniques enables add-dropfilters to be isolated whilst they are retuned from one channel toanother when the systems are being reconfigured. Application of suchtechniques results in increased insertion loss associated with add-dropfilters included within the systems, the insertion loss increasing asthe number of channels conveying communication traffic is increased.

A second approach employed in contemporary systems incorporatingadd-drop filters is for the add-drop filters to include a number oftuneable optical gratings which are tunable from a wavelengthintermediate between two neighbouring channels to a given channel. Thisapproach provides a characteristic that filters in the systems are nottuned through a number of channels before reaching their selectedchannel. However, the approach requires there to be provided a gratingfor each channel used in the systems, there arising thereby a problemthat insertions loss associated with add-drop filters in the systemsincreases as the number of channels is increased.

There is a further disadvantage that, when the first and secondapproaches are adopted, add-drop filters are designed for accommodatinga specific maximum number of channels; such a maximum number means thatthe add-drop filters have to be replaced if the number of channels usedin the systems are increased by system upgrades to more than the maximumnumber.

The inventors have appreciated that there is a need for a wavelengthselective optical filter capable of incorporation into add-drop filtersof communication systems that can tune directly from a first channel toa second channel without tuning through channels intermediate betweenthe first and second channels. Moreover, the inventors have appreciatedthat the optical filter should be tunable over a relatively large numberof channels so that the filters do not need to be replaced whencommunication system upgrades are implemented.

According to a first aspect of the present invention, there is provideda wavelength selective optical filter for receiving input radiation andoutputting corresponding filtered output radiation, characterised inthat the filter includes a plurality of mutually independently tunableoptical resonators for filtering the input radiation to generate theoutput radiation, the resonators being at least partially mutuallycoupled, and the resonators having associated therewith tuning rangeswhich at least partially mutually overlap.

The filter provides the advantage that it is capable of being tuned fromone wavelength to another without tuning through intermediatewavelengths therebetween.

The filter of the invention is distinguished from prior art filtersincorporating micro-tuned resonators in that the filter incorporatescavities which are mutually coupled. Such mutual coupling provides amore selective response than merely cascading filters as currently donein the art.

Conveniently, the filter in use is:

(a) at least partially transmissive to the input radiation to generatethe output radiation when the optical resonators are mutually tuned to asimilar wavelength; and

(b) substantially non-transmissive to the input radiation when theresonators are mutually detuned.

Such a filter provides the desirable characteristic that it issubstantially non-transmissive to radiation whilst being retuned fromwavelength to another.

Preferably, the coupling from one of the resonators to another resonatoradjacent thereto is in a range of 0.01 to 0.1% to obtain a useabledegree of selectivity from the filter.

Advantageously, the resonators include first and second tunableFabry-Perot cavities, the cavities being at least partially mutuallycoupled through a component common to the cavities. The cavities providea resonance characteristic when there are an integer number of halfwavelengths of radiation propagating between mirrors of the cavities.

Conveniently, the component is a second mirror assembly spatiallylocated between the cavities, the first and second cavities havingassociated therewith first and third mirror assemblies respectively, thefirst and second assemblies defining the first cavity, and the secondand third assemblies defining the second cavity. The second mirrorassembly provides a degree of mutual coupling between the cavities toprovide the filter with its wavelength selective response.

When implementing the filter, each cavity preferably includes a void ina region between its associated mirror assemblies. Advantageously, thevoid is in a range of 10 to 20 μm wide in a direction normal to majorplanes of its associated mirror assemblies, although 14 μm is itspreferred width.

In order to improve response of the filters, the reflectors arepreferably distributed Bragg reflectors. Such reflectors can be providedby each assembly comprising multi-layer structures.

AlGaAs, GaAs and AlAs will be used hereafter as abbreviations foraluminium gallium arsenide, gallium arsenide and aluminium arseniderespectively.

Advantageously, each multilayer structure comprises a plurality ofalternating layers of AlGaAs and aluminium oxide. Conveniently, thealuminium oxide layers each have a thickness in a range of 300 nm to 350nm, although 314.5 nm is a preferred thickness.

Moreover, the AlGaAs layers of the first and third assemblies are suchthat each layer beneficially comprises first, second and thirdsub-layers so that:

(i) the first and third sub-layers have a composition Al_(a)Ga_(b)Aswhere a is in a range of 0.58 to 0.62, and b is in a range 0.38 to 0.42;and

(ii) the second sub-layer has a composition Al_(c)Ga_(d)As where c is ina range of 0.28 to 0.32, and d is in a range of 0.68 to 0.72.

Incorporation of the sub-layers enables optical characteristics of theassemblies to be closely controlled in manufacture.

Furthermore, in the third assembly, the first and third sub-layers arepreferably each in a range of 10 to 20 nm thick, and the secondsub-layer is in a range of 90 to 100 nm thick. Additionally, in thefirst assembly, the first and third sub-layers are preferably each in arange of 45 to 55 nm thick, and the second sub-layer is preferably in arange of 15 to 25 nm thick. However, 52.8 nm and 19.9 nm are preferredspecific thicknesses for these layers.

Advantageously, the second assembly comprises aluminium oxide and AlGaAslayers such that each AlGaAs layer has a composition Al_(a)Ga_(b)Aswhere a is in a range of 0.58 to 0.62, and b is in a range 0.38 to 0.42.Preferably, in the second assembly, the AlGaAs layers are in a range of115 to 140 nm thick. Such layer thicknesses and composition assist toprovide a satisfactory assembly reflection characteristic for thefilter.

Beneficially, each cavity is tunable by altering a spatial separationbetween its respective mirror assembly and the second mirror assembly.In order to achieve such tuning, at least part of the mirror assembliescan be resiliently suspended and their mutual spatial separation can bealterable by applying piezo-electric forces to the mirror assemblies.Alternatively, at least part of the mirror assemblies can be resilientlysuspended and their mutual spatial separation can be alterable byapplying electrostatic forces to the mirror assemblies; the mirrorassemblies can be electrically connected to enable potential differencesto be applied therebetween to generate the electrostatic forces.

In a practical implementation of the filter, the second and third mirrorassemblies each advantageously comprise a central mirror regionsuspended on a plurality of compliant arms. In one embodiment of theinvention, the central mirror region is substantially circular andsuspended on four arms. Conveniently, the central region has aneffective diameter in a range of 50 to 150 μm and each arm has a lengthin a range of 600 to 2000 μm, although an effective diameter of 100 μmand a length of 1000 μm are specific preferred dimensions.

For ease of making electrical connections to the filter, the cavitiesand their associated mirrors can be fabricated onto a substrate interraced formation to enable electrical connection to be made to themirror assemblies, the assemblies being mutually electrically isolated.

Advantageously, the filter is fabricated using gallium arsenide orsilicon fabrication techniques.

According to a second aspect of the invention, there is provided anadd-drop filter for receiving input communication radiation and operableto drop and add radiation corresponding to a specific channel in theinput communication radiation, the add-drop filter incorporating afilter according to the first aspect of the invention for isolatingradiation corresponding to the channel.

According to a third aspect of the present invention, there is provideda method of fabricating a filter according to the first aspect of theinvention, the method comprising the steps of:

(a) forming a series of layers on a substrate, the layers forming mirrorassemblies and spacer layers therebetween;

(b) defining features in the layers corresponding to suspendedreflectors and associated compliant support arms;

(c) processing the substrate and its associated layers by etchingprocesses to generate the mirror assemblies and optical cavitiestherebetween where the spacer layers are present;

(d) defining and generating features on the assemblies operable toactuate the mirror assemblies relative to one another; and

(e) mounting the filter produced by steps (a) to (d) above in a carrierand making electrical connection to the filter.

Advantageously, the layers are formed by metal oxide chemical vapourdeposition (MOCVD). Moreover, the spacer layers are preferentially wetetched to form the optical cavities and render the mirror assembliesfreely suspended.

Conveniently, in the method, the mirror assemblies each comprisealternate layers of AlGaAs and AlAs, the AlAs subsequently heatprocessed to form aluminium oxide. Such alternate layers provideoptimised optical properties for the mirror assemblies.

According to a fourth third aspect of the present invention, there isprovided a method of tuning a filter according to the first aspect ofthe invention from a first wavelength to a second wavelength, the methodcomprising the steps of:

(a) tuning the resonators to the first wavelength so that the filterprovides selective filtration at the first wavelength;

(b) detuning the resonators from the first wavelength by tuning at leastone resonator in a first wavelength direction and another resonator inanother wavelength direction opposite to the first wavelength direction;

(c) tuning the resonators in a mutually detuned state towards the secondwavelength; and

(d) tuning the resonators finally to the second wavelength so that thefilter provides selective filtration at the second wavelength.

Embodiments of the invention will now be described, by way of exampleonly, with reference to the following drawings in which:

FIG. 1 is a schematic diagram of a prior art add-drop filterillustrating its channel isolation function;

FIG. 2 is a schematic diagram of a prior art communication systemincorporating three add-drop filters similar to the filter in FIG. 1;

FIG. 3 is an illustration of a wavelength selective optical filteraccording to the invention comprising a filter module connected to acirculator to provide an optical drop function for the add-drop filterin FIG. 1;

FIG. 4 is a schematic of a filter device included in the filter moduleillustrated in FIG. 3;

FIG. 5 is a graph of transmission and reflection characteristics of thefilter device in FIG. 4;

FIG. 6 is a graph of transmission characteristics of the filter devicein FIG. 4 when tuning from one wavelength to another;

FIG. 7 is a graph of attenuation and reflectance through the filterdevice in FIG. 4 as it is detuned; and

FIGS. 8 and 9 are illustrations of fabrication steps required forfabricating the filter device in FIG. 4.

Referring to FIG. 1, there is shown a prior art add-drop filterindicated by 10. The filter comprises an input port (IN), a through port(THROUGH), a drop port (DROP) and an add port (ADD); all the ports areadapted to interface to optical fibre waveguides. The input port isoperable to receive input radiation S_(in) and the through port isoperable to provide output radiation S_(out).

The filter 10 is designed to operate over a range of wavelengths whichaccommodates the radiation S_(in). The radiation S_(in) is a summationof radiation components associated with a sequence of channels C_(i)where an index i is an integer in a range of 1 to n which individuallyidentifies each channel, there being n channels in total in theradiation S_(in); for example, the radiation S_(in) has a wavelength inthe order of 1550 nm with the channels spaced at wavelength intervals of0.8 nm. The channels C_(i) monotonically change in wavelength accordingto their respective channel number index i.

Operation of the filter 10 will now be described with reference to FIG.1. The input radiation S_(in) propagates to the input port (IN) andfurther therefrom into the filter 10 whereat a radiation componentcorresponding to a channel C_(x) is extracted from the radiation S_(in)and output to the drop port (DROP). The radiation S_(in) minuscomponents corresponding to the channel C_(x), namely modified radiationS′_(in), propagates further into the filter 10 whereat a radiationcomponent corresponding to a channel C′_(x) input to the add port (ADD)is added to the radiation S′_(in) to yield the radiation S_(out) whichis then output at the through port (THROUGH). Thus, the output radiationS_(out) corresponds to the input radiation S_(in) except that thecomponent of radiation corresponding to the channel C_(x) in the inputradiation is replaced by a component of radiation corresponding to thechannel C′_(x) in the output radiation.

A conventional communication system incorporates a number of filterssimilar to the add-drop filter 10; such a conventional system isindicated by 100 in FIG. 2. The system 100 includes a multiplexer unit110, a demultiplexer unit 120 and three add-drop filters 130, 140, 150connected in series and inserted in a communication path connecting themultiplexer unit 110 to the demultiplexer unit 120. Each add-drop filter130, 140, 150 is of an identical design to the filter 10. Moreover, thefilters 130, 140, 150 are operable to filter channels C_(a), C_(b),C_(c) respectively where integer subscripts a, b, c can be mutuallydifferent and are included in a range of 1 to n.

The transmitter unit 110 comprises a series of optical inputs TC₁ toTC_(n) for receiving optical radiation corresponding to the channels C₁to C_(n) respectively. In a similar manner, the receiver unit 120comprises a series of optical outputs RC₁ to RC_(n) corresponding to thechannels C₁ to C_(n).

Operation of the system 100 will now be described. The multiplexer unit110 multiplexes the inputs TC to corresponding wavebands in outputoptical radiation K₁. The radiation K₁ propagates from the unit 110 tothe filter 130 which filters out a component of the radiation K₁corresponding to the channel C_(a) and outputs the component at its dropport. The filter 130 also adds radiation input to its add port to aportion of the radiation K₁ propagating through the filter 130 togenerate output radiation K₂. The radiation K₂ propagates to the filter140 whereat a component of radiation corresponding to the channel C_(b)is isolated and output it at its drop port. In a similar manner to thefilter 130, the filter 140 also adds radiation input to its add port toa portion of the radiation K₂ propagating through the filter 140 togenerate output radiation K₃. The radiation K₃ propagates to the filter150 whereat a component of radiation corresponding to the channel C_(c)is isolated and output it at its drop port. The filter 150 also addsradiation input to its add port to a portion of the radiation K₃propagating through the filter 150 to generate output radiation K₄. Theoutput radiation K₄ propagates to the demultiplexer unit 120 whereat itis demultiplexed to generate output optical radiation at the outputs RC.The radiation at the outputs RC correspond to those at the inputs TCexcept for the radiation input at inputs TC_(a), TC_(b), TC_(c) onaccount of the action of the filters 130, 140, 150. Additionalcomponents (not shown) are connected to the filters 130, 140, 150 forprocessing radiation isolated at the filters and for generatingradiation to be input to the add ports of the filters 130, 140, 150.

The inventors have appreciated that it is highly desirable for thesystem 100 to be reconfigurable so that the subscripts a, b, c can bealtered without interrupting traffic flow from the multiplexer unit 110to the demultiplexer unit 120. Such reconfiguration of the system 100can be achieved by the add-drop filters 130, 140, 150 incorporatingtherein wavelength selective optical filters, each filter tunable from afirst channel to a second channel without tuning through channelsintermediate between the first and second channels and operable toprovide add-drop functions for their associated add-drop filter.

Referring now to FIG. 3, there is indicated by 200 a wavelengthselective optical filter according to the invention operable to providean optical drop function for its associated add-drop filter 10. Thefilter 200 comprises a circulator 210 and a filter module 220, themodule 220 shown included within a dotted line 225. The filter module220 comprises an input lens 230, a tuneable filter device 240 and anoutput lens 250. The filter 200 incorporates an input port connectedthrough an optical fibre 260 to an input port J₁ of the circulator 210.Moreover, the circulator 210 comprises an output port J₃ which isconnected through an optical fibre 270 to an output port of the filter200. Furthermore, the circulator 200 comprises a further port J₂ whichis connected through an optical fibre 280 to the input lens 230. Theoutput lens 250 is connected through an optical fibre 290 to a drop portof the filter 200.

Operation of the filter 200 will now be described with reference to FIG.3. The input port of the filter 200 receives the radiation S_(in)applied to the IN port of the filter 10. The radiation S_(in) propagatesalong the fibre 260 to the input port J₁ of the circulator 210. Theradiation S_(in) propagates within the circulator 210 to the port J₂ atwhich it is output to propagate along the fibre 280 and through the lens230 to the filter device 240; the lens 230 forms a beam having adiameter in a range of 50 to 100 μm which is received by the device 240.If the beam received by the device 240 is broader than 100 μm,deterioration in the filter 200 response will result. The device 240 istuned to a channel C_(x) where an integer index x is in a range of 1 ton. Radiation components in the radiation S_(in) corresponding to thechannel C_(x) propagate through the device 240 and are received at thelens 250 through which they propagate onwards and through the fibre 290to the drop port of the filter 200. Radiation components in theradiation S_(in) corresponding to the channels C₁ to C_(x−1) and C_(x+1)to C_(n) are reflected from the device 240 and propagate back throughthe lens 230 and the fibre 280 to the port J₂ of the circulator 210;these components propagate further in the circulator 210 to its outputport J₃ and further therefrom along the fibre 270 to the output port ofthe filter 200.

The filter 10 includes first and second filter units, the first unitcorresponding to the filter 200 and the second unit corresponding to amodified version of the filter 200 adapted for injecting radiationcomponents corresponding to the channel C_(x).

The filter device 240 will now be described in further detail withreference to FIG. 4. The device 240 comprises a thinned gallium arsenidesubstrate 300 having a thickness in a range of 150 μm to 250 μm,although 200 μm is a preferred thickness. The substrate 300 comprises afirst major face indicated by 310 which is coated in an anti-reflectioncoating 305 operable to counteract reflection from the face 310 atinfra-red radiation wavelengths in the order of 1500 nm. The substrate300 comprises a second major face indicated by 320 on an opposite sideof the substrate 300 to the first face 310. The second face 320 hasfabricated thereon a first mirror assembly 330 comprising alternatelayers of AlGaAs and aluminium oxide, namely four layers of AlGaAs andthree layers of aluminium oxide. Each of the aluminium oxide layers isin a range of 300 to 350 nm thick, although 314.5 nm is its preferredthickness. Each AlGaAs layer comprises three sequential sub-layers,namely first, second and third sub-layers such that:

(i) the first and third sub-layers have a composition Al_(a)Ga_(b)Aswhere a is in a range of 0.58 to 0.62, and b is in a range 0.38 to 0.42;and

(ii) the second sub-layer has a composition Al_(c)Ga_(d)As where c is ina range of 0.28 to 0.32 and d is in a range of 0.68 to 0.72.

A preferred composition for the first and third sub-layers isAl_(0.6)Ga_(0.4)As, and a preferred composition for the second sub-layeris Al_(0.3)Ga_(0.7)As. The first and third sub-layers are each in arange of 45 to 55 nm thick, and the second sub-layer is in a range of 15to 25 nm thick. However, 52.8 nm is a preferred thickness for each ofthe first and third sub-layers, and 19.9 nm is a preferred thickness forthe second sub-layer.

Above the mirror assembly 330 remote from the substrate 300 is a firstcavity 340 of height in a range of 10 to 20 μm in a direction normal tothe major faces 310, 320 of the first assembly 330, although 14 μm is apreferred height. Suspended above the first assembly 330, and separatedtherefrom by the first cavity 340, is a second mirror assembly indicatedby 350.

The second assembly 350 is unitary and incorporates a centralsubstantially circular mirror 360 suspended on four arms, for example anarm 370, from a peripheral region 380 of the assembly 350, theperipheral region 380 connected to the first assembly 330 by way of afirst relatively thick layer of GaAs, the first relatively thick layerbeing substantially 14 μm thick.

The central mirror 360 and its associated arms are fabricated by formingfour holes, for example a hole 390, into the mirror assembly 350. Thus,the central mirror 360, its associated arms and the peripheral region380 are all of unitary construction. The central mirror 360 has aneffective diameter in a range of 50 to 150 μm, although 100 μm is itspreferred diameter. Each arm has a length in a range of 600 to 2000 malthough 1000 μm is its preferred length. The arms each have a lateralwidth in a range of 15 to 30 μm, although 20 μm is their preferredwidth.

The second assembly 350, in a similar manner to the first assembly 330,is a multilayer structure comprising alternate layers of AlGaAs andaluminium oxide. Each of the aluminium oxide layers is in a range of 300to 350 nm thick, although 314.5 nm is its preferred thickness. Likewise,each of the AlGaAs layers is in a range of 115 to 140 nm thick, although125.0 nm is a preferred thickness. The AlGaAs layers have a compositionAl_(a)Ga_(b)As where a is in a range of 0.58 to 0.62, and b is in arange 0.38 to 0.42; however, Al_(0.6)Ga_(0.4)As is a preferredcomposition for the layers.

In the second assembly, there are six aluminium oxide layers and sevenAlGaAs layers.

Above the mirror assembly 350 remote from the substrate 300 is a secondcavity 400 of height in a range of 10 to 20 μm in a direction normal tothe major faces 310, 320 of the first assembly 330, although 14 μm is apreferred height. Suspended above the second assembly 350, and separatedtherefrom by the second cavity 400, is a third mirror assembly indicatedby 420.

The third assembly 420 includes a substantially circular central mirror430 suspended on four arms, for example an arm 440, from a peripheralregion 450. The mirror 430, the arms and peripheral region 450 areunitary parts of the assembly 420. Moreover, the mirror 430 and the armsof the assembly 420 are of similar lateral dimensions to the mirror 360and the arms of the first assembly 330. Furthermore, the mirrors 360,430 are mutually aligned along an axis normal to the plane of the firstand second assemblies 330, 350. The peripheral region 450 is connectedto the peripheral region 380 of the second assembly 350 by way of asecond relatively thick layer of GaAs between the peripheral regions380, 450, the layer having a thickness in a range of 10 to 20 μmalthough substantially 14 μm is a preferred thickness.

The third assembly 420, in a similar manner to the first and secondassemblies 330, 350, is a multilayer structure comprising alternatelayers of AlGaAs and aluminium oxide. Each of the aluminium oxide layersis in a range of 300 to 350 nm thick, although 314.5 nm is its preferredthickness.

Each AlGaAs layer of the third assembly 420 comprises three sequentialsub-layers, namely first, second and third sub-layers such that:

(i) the first and third sub-layers have a composition Al_(a)Ga_(b)Aswhere a is in a range of 0.58 to 0.62, and b is in a range 0.38 to 0.42;and

(ii) the second sub-layer has a composition Al_(c)Ga_(d)As where c is ina range of 0.28 to 0.32, and d is in a range of 0.68 to 0.72.

A preferred composition for the first and third sub-layers isAl_(0.6)Ga_(0.4)As, and a preferred composition for the second sub-layeris Al_(0.3)Ga_(0.7)As.

The first and third sub-layers are each in a range of 10 to 20 nm thick,and the second sub-layer is in a range of 90 to 100 nm thick. However,13.5 nm is a preferred thickness for each of the first and thirdsub-layers, and 94.9 nm is a preferred thickness for the secondsub-layer.

The first and second cavities 340, 400 enable the mirrors 360, 430 to befreely suspended on their respective arms, a mutual spacing between themirrors 360, 430 and the first assembly 330 being adjustable by applyingforces to the mirrors 360, 430. Such forces can be electrostatically orpiezo-electrically generated. Adjustment of such forces enables thedevice 240 to be tuned.

Moreover, the relatively complex arrangement of layers in the assemblies330, 350, 420 is chosen to allow precise adjustment of layer refractiveindex and hence overall central mirror 360, 430 and first assembly 330reflectivity.

The peripheral regions 380, 450 are arranged in terraced formation asshown in FIG. 4 in order to facilitate electrical connection by wirebonding to connection pads 460, 470, 480 associated with the assemblies330, 350, 420 respectively. The relatively thick GaAs layers areoperable to substantially mutually electrically isolate the assemblies330, 350, 420. Moreover, when electrostatic forces are to be used toactuate the central mirrors 360, 430 relative to one another and thefirst assembly 330, the assemblies 330, 350, 420 are sufficientlyconductive so that electrical potentials applied to the pads 460,470,480 through associated wires bonded thereto control potentials ofthe central mirrors 360, 430 relative to the first assembly 330.

Operation of the filter device 240 will now be described with referenceto FIGS. 3 and 4. Incoming radiation propagating along the fibre 280from the circulator 210 is focussed by the lens 230 onto the centralmirror 430 of the third assembly 420. The central mirror 430 ispartially transmissive and radiation incident thereupon propagates intothe second cavity 400 and is largely reflected at the central mirror 360back to the central mirror 430. If the radiation has components of awavelength such that the effective distance between the central mirrors360, 430 is an exact number of half wavelengths, resonance within thesecond cavity 400 occurs and the components give rise to standing wavesin the second cavity. Alternatively, if the radiation has components ofa wavelength such that the effective distance between the centralmirrors 360, 430 is not an exact number of half wavelengths, no standingwaves are formed. The second cavity 400 is weakly coupled through thecentral mirror 360 to the first cavity 340 and vice versa such thatcoupling from one cavity to another is in a range of 0.01 to 0.1%. Whenthe first cavity 340 is also tuned to the same wavelength as the secondcavity 400, resonance in the first cavity 340 results in an efficientcoupling of radiation components corresponding to resonance from thesecond cavity 400 into the first cavity 340. The first assembly 330 ispartially transmissive to radiation so that components of radiation atresonance in the first cavity 340 are transmitted through the firstassembly 330 and through the substrate 300 and its antireflectioncoating 305 to propagate to the lens 250 and further along the fibre 290to the drop port.

Components of radiation corresponding to resonance of the cavities 340,400 are thereby transmitted through the device 240 whereas components ofradiation not corresponding to resonance are reflected from the secondand third assemblies 350, 420 back to the circulator 210. Transmissionthrough the device 240 substantially only occurs when the cavities 340,400 are tuned to a mutually similar wavelength so that their resonancescorrespond.

Referring now to FIG. 5, there is shown a graph of transmission andreflection characteristics of the filter device 240 in FIG. 4 when thecavities 340, 400 are tuned to a similar resonance wavelength, namely1550 nm. In the graph, a curve 500 depicts radiation transmissionthrough the device 240 and a dashed curve 510 depicts radiationreflection from the device 240. Radiation attenuation through the device240 at resonance are less than 1 dB although coupling losses associatedwith the lenses 230, 250 and the fibres 280, 290 result in a overallinsertion loss between the circulator 210 and the drop port in a rangeof 1 to 2 dB. The graph shows that reflection loss from the device 240at resonance is at a level of −42 dB when its transmissive attenuationloss is less than 1 dB. Moreover, reflection loss exhibited by thedevice 240 is less than 1 dB when its transmission loss is at a level of−25 dB at a wavelength difference of 0.8 nm from resonance at awavelength of 1550 nm. At wavelength differences of more than 0.8 nmfrom resonance in a wavelength range of 1545 nm to 1555 nm shown in FIG.5, transmission losses through the device 240 are in excess of 25 dB.

Operation of the device 240 switching from one channel C_(i) to anotherchannel will now be described with reference to FIG. 6. In FIG. 6, thereis shown three mutually orthogonal axes, namely a wavelength axis 600, atime axis 610 and a transmittance axis 620. Directions of increasingwavelength, time and transmittance are indicated by arrows 640, 650, 660respectively. Within the axes 600, 610, 620 is included a 3-dimensionalcurve indicated by 700 of transmittance of the device 240 depending upontime and wavelength. The curve 700 is projected as a 2-dimensional curveindicated by 720 at a rear face of the graph; this curve 720 illustrateswavelength versus transmittance.

A first peak 740 corresponds to the device 240 tuned to one of thechannels C_(i) with nominal wavelength λ₁ and a second peak 820corresponds to the device 240 tuned to another of the channels withnominal wavelength λ₂. Initially, the cavities 340, 400 of the device240 are tuned to the wavelength λ₁. In order to retune the cavities 340,400, a voltage difference applied between the second and thirdassemblies 350, 420 relative to a potential of the first assembly 330 isadjusted to detune both cavities 340, 400 in mutually oppositewavelength directions from the wavelength λ₁. This mutual detuningcauses the first peak 740 to broaden and reduce in transmissivity asdepicted by curves indicated by 760, 780, thereby effectively switchingoff a transmission function provided by the device 240. The two cavities340, 400 are then tuned, by moving their respective central mirrors 360,430 by substantially equal amounts by altering a potential differencebetween the assemblies 350, 420 relative to a potential the firstassembly 330, towards the second wavelength λ₂ as depicted by curvesindicated by 800, 810. When the cavities 340, 400 are both tuned to thewavelength λ₂, the device 240 again becomes transmissive as representedby the peak 820.

In the curve 720, it can be seen that the device 240 is substantiallynon-transmissive between the peaks 740, 820 thereby tuning from onechannel to another without tuning through intermediate channelstherebetween; in FIG. 5, the wavelengths λ₁ and λ₂ correspond to two ofthe channels C_(i).

The device 240 is also capable of functioning as a controlled attenuatorby selectively slightly mutually detuning one or more of its cavities340, 400. Thus, the device 240 can provide wavelength selectiveattenuation depending upon to which channel the cavities 340, 400 arenominally tuned. A voltage difference is applied to the assemblies 350,420 relative to the first assembly 330 can be used to determine a degreeof detuning and thereby determine attenuation exhibited by the device240. When performing an attenuating function, variation in reflectivityof the device can be utilised. FIG. 7 illustrates a graph of reflectiveattenuation provided by the device 240 included in the filter 200 fromthe fibre 280 to the fibre 290 when its cavities 340, 400 are detuned toa mutual wavelength difference of 1.6 nm, namely each cavity detunedfrom nominally 1550 nm by an amount of 0.8 nm. The attenuation providedby the device 240 in the filter 200 is shown to be controllable fromsubstantially −15 dB to −3 dB.

The device 240 operating as a controlled attenuator is especially usefulin communication networks where differential losses between channelsaccumulate through the networks and can degrade network performance.Thus, the device 240 can be used to assist with equalising power levelsacross the channels C_(i) in the networks.

Fabrication of the device 240 will now be described. FIGS. 8 and 9 areillustrations of fabrication steps of a method of fabricating the filterdevice 240 in FIG. 4, the method comprising STEP 1 to STEP 6.

Step 1:

Reference is made to FIG. 8. Initially, the gallium arsenide substrate300 is polished on both its major faces to a mirror finish and therebythinned to a thickness of substantially 200 μm. On the major surface 320of the substrate 300, the first assembly 330 is grown onto the substrate300 by metal oxide chemical vapour deposition (MOCVD) in an evacuateddeposition apparatus. The first assembly 330 is a distributed Braggreflector (DBR) and comprises alternate layers of AlGaAs and AlAs. Thereflector of the first assembly 330 includes four layers of AlGaAs andthree layers of AlAs. The AlAs layers are each 314.5 nm thick. Moreover,the AlGaAs layers each comprise three sequential sub-layers, namely afirst sub-layer of a nominal thickness 52.8 nm and a nominal compositionof Al_(0.6)Ga_(0.4)As, a second sub-layer of a nominal thickness 19.9 nmand a nominal composition Al_(0.3)Ga_(0.7)As, and a third sub-layer of athickness and a composition similar to the first sub-layer. The firstassembly 330 as a result has a thickness in the order of 1.5 μm.

Next, a first spacer layer of gallium arsenide is grown onto the firstassembly 330. The spacer layer is nominally of 14 μm thickness.

The second assembly 350 is then grown by MOCVD onto the first spacerlayer remote from the substrate 300, the second assembly 350 also beinga DBR. The reflector of the second assembly 350 includes seven layers ofAlGaAs and six layers of AlAs. The AlAs layers are each 314.5 nm thick.Moreover, the AlGaAs layers are each of a nominal thickness 125.0 nm anda nominal composition of Al_(0.6)Ga_(0.4)As. The second assembly 350 hasa thickness in the order of 2.8 μm.

Next, a second spacer layer of gallium arsenide is grown onto the firstassembly 350. The spacer layer is nominally of 14 μm thickness.

Finally, the third assembly 420 is then grown by MOCVD onto the secondspacer layer remote from the substrate 300, the third assembly 420 alsobeing a DBR. The reflector of the third assembly 350 includes threelayers of AlGaAs and two layers of AlAs. The AlAs layers are each 314.5nm thick. Moreover, the AlGaAs layers each comprise three sequentialsub-layers, namely a first sub-layer of a nominal thickness 13.5 nm anda nominal composition of Al_(0.6)Ga_(0.4)As, a second sub-layer of anominal thickness 94.9 nm and a nominal composition Al_(0.3)Ga_(0.7)As,and a third sub-layer of a thickness and a composition similar to thefirst sublayer. The third assembly 420 has a thickness in the order of1.1 μm.

Completion of STEP 1 results in the generation of a workpiece indicatedby 880.

Step 2:

Reference is made to FIG. 9. A layer 900 of photoresist is then spunonto the third assembly 420 of the workpiece 880 and photolithographicand associated resist development techniques applied to define windowsin the layer 900. The windows are useable for delineating the centralmirrors 360, 430 and their associated arms in STEP 2.

Step 3:

The workpiece 880 from STEP 2 is then subjected to anisotropic reactiveion etching (RIE) or chemically assisted ion beam etching (CAIBE) wherethe layer 900 provides a stencil for etching. Etching is continued untilholes 910, 920 through the MOCVD-deposited layers are produced whichreach down through the MOCVD layers beyond the first assembly 330.

Step 4:

The workpiece 880 from STEP 3 is then subjected to a steam environmentat a temperature in a range of 380 to 420° C., although 400° C. is apreferred temperature, which oxidises the AlAs layers of the workpiece880 to aluminium oxide. The workpiece is next subjected to wetpreferential etching which does not etch the assemblies 330, 350, 420but removes part of the spacer layers in the vicinity of the holes 910,920 to leave the central mirrors 360, 430 and their associated armsfreely suspended and the cavities 340,400 defined. Moreover, the wetetching also removes residual traces of the first spacer layer remainingon the first assembly 330 in the first cavity 340 beneath the firstcentral mirror 360.

Step 5:

Further lithographic, RIE and preferential wet etching techniques arethen applied to generate a terraced profile for the workpiece 880 fromstep 4 as indicated by 930. This terraced profile assists with makingelectrical connection to the workpiece using standard wire bondingequipment. Metal is then selectively deposited onto the assemblies 330,350, 420 to form the pads 460, 470, 480 to which connection wires are tobe bonded.

Step 6:

The workpiece 880 from STEP 5 is next mounted in a suitable carrier (notshown) allowing infra-red radiation propagation through the workpiece880, the anti-reflection coating 305 applied on the major face 310 ofthe substrate 300 and then wires bonded to the pads 460, 470, 480 tocomplete fabrication of the workpiece 880 to provide the device 240.

Several devices identical to the device 240 can be fabricated from onesubstrate which is cleaved after STEP 5 to provide individual devicesfor packaging and wire bonding.

It will be appreciated that modifications can be made to the filter 200and the device 240 without departing from the scope of the invention.For example, the device 240 can be modified to incorporate more than twomutually coupled resonant cavities to obtain a more wavelength selectivefiltration response. Alternatively, several devices 240 can be cascadedin series to provide a more selective response. Furthermore, it shouldbe noted that techniques of detuning coupled optical resonators toachieve direct channel switching in add-drop filters as used in theinvention is also applicable to other types of mutually coupled opticalresonator other than micro-mechanically tuned optical Fabry-Perotresonators, for example mutually coupled optical ring resonators inwaveguide devices.

Although electrostatic actuation of the central mirrors 360, 430 isdescribed above, other methods of actuation are possible; for examplepiezo-electric actuation where piezo-electric layers, for examplecomprising zinc oxide as a piezo-electric material, are fabricated ontothe assemblies 330, 350, 420 in STEP 1 of the method of fabricationdepicted in FIG. 8. Such piezo-electric layers enable differentialstresses to be generated in arms supporting the central mirrors 360, 430thereby causes the mirrors to change their spatial separation relativeto the first assembly 330.

It will be further appreciated that feedback control can be applied tostabilise spatial separation of the mirrors 360, 430 relative to thefirst assembly 330 so that the device 240 remains stably tuned to itsallocated channel C_(i). Such feedback control can, for example, employlow frequency artefacts in channel radiation reflected or transmittedthrough the device 240, the low frequency artefacts being at channelmodulation frequencies less than that of communication traffictransmitted in the channels C_(i).

Although the aforementioned device 240 is fabricated using galliumarsenide fabrication techniques, the device 240 can also be fabricatedin modified form in silicon materials, such silicon materials havingsuperior mechanical properties compared to gallium arsenide which isrelatively brittle compared to silicon. The device 240 in modified formin silicon can employ a multipart construction, the parts bondedtogether after fabrication, for example by fusion bonding.Alternatively, the device 240 in modified form in silicon can befabricated as a surface structure on a silicon substrate, mirrors andarms of the device 240 in modified form being formed by epitaxialdeposition processes with sacrificial oxide layers for use in generatingvoids corresponding to optical cavities of the device.

What is claimed is:
 1. A method of fabricating a tuneable wavelengthselective optical filter including a plurality of at least partiallymutually coupled optical resonators in which a resonant wavelength ofeach resonator is independently tuneable, the method comprising thesteps of: a) forming a series of layers on a substrate, the layersforming mirror assemblies and spacer layers therebetween; b) definingfeatures in the layers corresponding to suspended reflectors andassociated compliant support arms; c) processing the substrate and itsassociated layers by etching processes to generate the mirror assembliesand optical cavities therebetween where the spacer layers are present;d) defining and generating features on the assemblies operable toactuate the mirror assemblies relative to one another; and e) mountingthe filter in a carrier and making electrical connection to the filter.2. The method according to claim 1, in which the layers are formed bymetal oxide chemical vapor deposition (MOCVD).
 3. The method accordingto claim 1, in which the spacer layers are preferentially wet etched toform the optical cavities and render the mirror assemblies freelysuspended.
 4. The method according to claim 1, in which the mirrorassemblies each comprise alternate layers of aluminum gallium arsenideand aluminum arsenide; and further comprising the step of heatprocessing the aluminum arsenide to form aluminum oxide.
 5. A method oftuning a tuneable wavelength selective optical filter from a firstwavelength range to a second wavelength range, the filter including aplurality of at least partially mutually coupled optical resonators inwhich a resonant wavelength of each resonator is independently tuneable,the method comprising the steps of: a) tuning the resonant wavelength ofeach of the resonators to the first wavelength range so that the filterprovides selective filtration at the first wavelength range; b) alteringthe resonant wavelength of at least one resonator to mutually detune theresonators; c) altering the resonant wavelength of each of theresonators in the mutually detuned state towards the second wavelength;and d) tuning the resonant wavelength of each resonator finally to thesecond wavelength range so that the filter provides selective filtrationat the second wavelength range.
 6. The method according to claim 5 inwhich the resonators are mutually detuned from the first wavelengthrange by tuning at least one resonator in a first wavelength directionand another in a direction opposite to the first wavelength direction.7. The method according to claim 6, and further comprising the step oftuning the resonators in their mutually detuned state towards the secondwavelength range.
 8. A method of selecting a selected channel fromwavelength division multiplex (WDM) radiation, said WDM radiationcomprising a plurality of channels each channel having associatedtherewith an allocated wavelength range, the method comprising the stepsof: a) providing a tuneable filter including a plurality of at leastpartially mutually coupled optical resonators in which a resonantwavelength of each resonator is independently tuneable; b) applying saidWDM radiation to an input of said filter; c) tuning the resonantwavelength of each of the resonators to a first wavelength correspondingto a first selected channel so that the filter provides selectivefiltration at a first wavelength range corresponding to the firstselected channel; d) altering the resonant wavelength of at least oneresonator to mutually detune the resonators in a mutually detuned state;e) altering the resonant wavelength of each of the resonators in themutually detuned state towards a second wavelength corresponding to asecond selected channel; and f) tuning the resonant wavelength of eachresonator finally to the second wavelength so that the filter providesselective filtration at a second wavelength range corresponding to thesecond selected channel.
 9. The method according to claim 8, andcomprising tuning the resonators in their mutually detuned state throughany channels lying in wavelength between the first and second selectedchannels.
 10. The method according to claim 8, and comprising mutuallydetuning the resonator from the first wavelength by tuning at least oneresonator in a first wavelength direction and another in a directionopposite to the first wavelength direction.
 11. The method according toclaim 10, and comprising tuning the resonators in their mutually detunedstate towards the second wavelength.
 12. A tuneable wavelength selectiveoptical filter, comprising: a plurality of at least partially mutuallycoupled optical resonators, each resonator comprising a Fabry-Perotcavity defined between partially reflecting planes and having associatedtherewith a resonant wavelength, the cavities being mutually coupled toeach other through a reflecting plane common to the cavities, whereinthe filter is at least partially transmissive when the resonantwavelength of each cavity is substantially the same, the filter beingtransmissive at a wavelength range corresponding to the resonantwavelength and substantially non-transmissive when the resonantwavelengths of each cavity are not substantially the same, and whereinat least one reflecting plane associated with each cavity isindependently moveable to enable a separation between the planes to beindependently alterable, thereby enabling the resonant wavelength ofeach cavity to be independently tuned.
 13. The tunable filter accordingto claim in which the common reflecting plane is independently moveable.14. The tuneable filter according to claim 12, in which mutual couplingfrom one resonator to an adjacent resonator is in a range of 0.01 to0.1%.
 15. The tuneable filter according to claim 12, in which eachreflecting plane comprises a respective mirror assembly, wherein a firstcavity is defined between a first and a second mirror assembly and asecond cavity is defined between the second mirror assembly and a thirdmirror assembly, the second mirror assembly being spatially locatedbetween the first and third mirror assemblies and independentlymoveable.
 16. The tuneable filter according to claim 15, in which theassemblies each comprise multi-layer structures.
 17. The tuneable filteraccording to claim 15, in which each multilayer structure comprises aplurality of alternating layers of aluminum gallium arsenide andaluminum oxide.
 18. The tuneable filter according to claim 17, in whichthe aluminum oxide layers each have a thickness in a range of 300 nm to350 nm.
 19. The tuneable filter according to claim 17, in which aluminumgallium arsenide layers of the first and third assemblies are such thateach layer comprises first, second and third sub-layers in which thefirst and third sub-layers have a composition Al_(a)Ga_(b)As where a isin a range of 0.58 to 0.62, and b is in a range 0.38 to 0.42; and thesecond sub-layer has a composition Al_(c)Ga_(d)As where c is in a rangeof 0.28 to 0.32, and d is in a range of 0.68 to 0.72.
 20. The tuneablefilter according to claim 19, in which, in the third assembly, the firstand third sub-layers are each in a range of 10 to 20 nm thick, and thesecond sub-layer is in a range of 90 to 100 nm thick.
 21. The tuneablefilter according to claim if in which, in the first assembly, the firstand third sub-layers are each in a range of 45 to 55 nm thick, and thesecond sub-layer is in a range of 15 to 25 nm thick.
 22. The tuneablefilter according to claim 17, in which the aluminum gallium arsenidelayers of the second assembly have a composition Al_(a)Ga_(b)As where ais in a range of 0.58 to 0.62, and b is in a range 0.38 to 0.42.
 23. Thetuneable filter according to claim 22, in which, in the second assembly,the aluminum gallium arsenide layers are in a range of 115 to 140 nmthick.
 24. The tuneable filter according to claim 15, in which thesecond mirror assembly and at least one other mirror assembly areresiliently suspended and their mutual spatial separation is alterableby applying piezo-electric forces to the mirror assemblies.
 25. Thetuneable filter according to claim 24, in which the resilientlysuspended mirror assemblies each comprise a central mirror regionsuspended on a plurality of compliant arms.
 26. The tuneable filteraccording to claim 25, in which each central mirror region issubstantially circular and suspended on four arms.
 27. The tuneablefilter according to claim 26, in which each central region has aneffective diameter in a range of 50 to 150 μm and each arm has a lengthin a range of 600 to 2000 μm.
 28. The tuneable filter according to claim15, in which the second mirror assembly and at least one other mirrorassembly are resiliently suspended and their mutual spatial separationis alterable by applying electrostatic forces to the mirror assemblies.29. The tuneable filter according to claim 28, in which the mirrorassemblies are electrically connected to enable potential differences tobe applied thereto to generate the electrostatic forces.
 30. Thetuneable filter according to claim 28, in which the resilientlysuspended mirror assemblies each comprise a central mirror regionsuspended on a plurality of compliant arms.
 31. The tuneable filteraccording to claim 30, in which each central mirror region issubstantially circular and suspended on four arms.
 32. The tuneablefilter according to claim 31, in which each central region has aneffective diameter in a range of 50 to 150 μm and each arm has a lengthin a range of 600 to 2000 μm.
 33. The tuneable filter according to claim28, in which the cavities and their associated mirrors assemblies arefabricated onto a substrate in terraced formation to enable electricalconnection to be made to the mirror assemblies, the assemblies beingsubstantially mutually electrically isolated.
 34. The tuneable filteraccording to claim 12, in which each cavity includes a void in a regionbetween associated reflecting planes.
 35. The tuneable filter accordingto claim 34, in which the voids are in a range of 10 to 20 μm wide in adirection normal to major planes of its associated reflectingplanes/mirror assemblies.